1. Field of the Invention
This invention relates to controlled atmosphere furnaces and more particularly it concerns novel arrangements whereby a fuel-fired radiant tube type controlled atmosphere furnace is adapted to electrical operation.
2. Description of the Prior Art
Controlled atmosphere furnaces are used primarily for heat treating and annealing. These furnaces are usually heated by the hot combustion products from a gas or oil burner positioned outside the furnace. These combustion products are recirculated through radiating tubes mounted inside the furnace; and the radiating tubes isolate these combustion products from the atmosphere within the furnace.
Fuel-fired industrial furnaces are often unsatisfactory from the standpoint of noise and pollution. Also, the radiating tubes in these furnaces sometimes develop "hot spots" due to improperly adjusted burners, which reduce the useful life of the radiant tubes.
In order to avoid these difficulties without completely replacng still functional furnaces, attempts have been made to convert existing fuel-fired radiant tube furnaces to electrical power. This was done by providing electrical resistance elements, e.g., silicon carbide elements, spirally wound wire, folded or corrugated or expanded strips; and mounting these elements inside the existing furnaces. Problems were encountered, however, due to the difficulty of properly supporting the electrical heating elements inside such furnaces without almost rebuilding the entire furnace. The heating elements were also subject to frequent breakage or short circuiting.
Other electrical heating elements which have been used in furnaces or ovens comprise electrical resistance elements which are supported on a dielectric material such as ceramic. The dielectric material in turn is encased in a radiating cover, usually made of metal. These heating elements however, are expensive to manufacture and they do not provide sufficient heat radiating capability for many applications, because of the temperature gradient between the actual heating wire and the radiating cover.